Nanodiamond particles and their aggregates may contain optically-active structures such as non-diamond carbon atoms, dopants (non-carbon atoms in substitutional or interstitial positions within the diamond lattice), point defects, linear defects, planar defects, interstitial carbon atoms and/or complexes of the above which exhibit weak photoluminescent properties. Examples of optical structures include, but are not limited to substitutional or interstitial nitrogen (N), nitrogen atom(s) complexes with vacancy(s) (V), substitutional or interstitial Si atom, Si atom(s) complexes with vacancy(s) and others.